Human milk has more lactose but less protein than cow or goat milk.
Man has been drinking the milk of cows, goats, sheep, baffalo, horses, and other mammals for a long time. All mammals produce milk. The first milk produced after the birth of an offspring looks different and has different composition. It is called colostrum. After a few days, a transition to white milk is occurs and it is called mature milk.

Colostrum - The first milk
About 100 ml of colostrum is secreted on the second and third days after birth by human mothers. Compared to the later mature milk, human colostrum is lemon-yellow and richer in protein and salts, but lower in fat and sugar. Colostrum is high in globulins and unlike mature milk is therefore heat coagulable. As much as 10% of the protein in cow colostrum is globulins and much of this globulin is immune globulins.

Newborn mammals acquire immunity to certain infections via the colostrum. This seems more pronouced in calves than human babies. Immune globulins are absent from the blood of calves, but immune globulins appear in calf bood within 3 hours after colstrum is fed. The passage of immune globulins from food to blood occurs in all mammals without destruction during the first day or two after birth. Normally all proteins are digested, but trypsin inhibitor in colostrum allows the immune globulins to reach the intestines without destruction. The globulins appear to pass from the intestine via the lymphatic system to the heart as fats do in later life. Amino acids and simple sugars pass from the intestine via the portal vein to the liver.

In human lactation, pure colostrum is not produced beyond the 5th day after birth. After a gradual transition only mature milk is produced after the 10th day. The fat content of human milk continues to rise for the first two weeks. Lactose increases for five weeks. In cows, the transition occurs from day 5 to day 12.

Mature Milk
The composition of milk varies from cow to cow and differs for the various breeds. However, commercial milks are a blend of many animals of many breeds and have the average compositions shown in this table.

Constituent

Human

Cow

Goat

Protein

1.2

3.3

3.4

Lactose

7.0

4.8

4.7

Fat

3.8

3.8

4.1

Ash (minerals)

0.21

0.71

0.77

Total solids

12.4

12.8

13.0

Lactose - Milk Sugar
Lactose is the only significant sugar found in milk. Lactose is made only by mammals. It is found only in milk. Lactose is a disaccharide consisting of one molecule of glucose and one molecule of glactose linked. This beta glysoside linkage is between carbon 4 of glucose and carbon 1 of glucose. Thus, the chemical name for lactose is D-gluco-pyranose-4-(beta-D-galactopyranoside). Only bacteria or organisms having a suitable beta-glucosidase are able to split this bond. This makes milk a useful diagnostic medium for identifying bacteria. Notice it is easy to remember that lactose is made from one molecule of glucose and one molecule of galactose => lactose. All the common double sugars contain one or more glucose molecule. Lactose is formed in the mammary gland from glucose. The lactose content of milk is not much affected by the maternal diet or level of blood glucose. Lactose is frequently detectable in human urine at birth and during lactation. The common disaccharides are shown in the the table below. While 3 of these are glucose-glucose, the linkages are different so different enzymes are required for their digestion.

Disaccharide

Sugar 1

Sugar 2

Natural Occurance

Lactose

glucose

galactose

Milk of mammals only

Sucrose

glucose

fructose

fruits, cane, maple, beet

Trehalose

glucose

glucose

yeasts and fungi

Maltose

glucose

glucose

digestion of starch by malt

Cellobiose

glusose

glucose

digestion of cellulose

Lactose is a reducing sugar and exhibits mutarotation because carbon 1 of the glucose moity is free. Like other disaccharides (double sugars), lactose reduces Cu++ solutions more slowly than monosaccharides. Therefore, Barfoed's reagent can distinguish lactose from simple sugars. Lactose yields a specific phenylosazone, a positive mucic acid test, and is not fermented by baking yeast (Saccharomyces cerviseaie ////(sp)////).

The percentage of lactose in milk varies with species. The percentage of lactose in human milk ranges from 6.5 to 7.5%. The average in cow and goat milk is less than 5% and the range is greater. When human babies are reared on cow or goat milk, it is customary to add lactose, sucrose, glucose, or malto-dextrin (partially hydrolyzed starch (maltose and dextrins)). Starch is a glucose polymer; therefore maltose (a disaccharide) is glucose-glucose.

Lactose is less soluble and less sweet than other common sugars. This lower sweetness may help babies drink more milk. In one small study, babies on a diet containing 36 grams of lactose per day retained more calcium than babies getting glucose as the sugar.

Lactose is converted to lactic acid by the normal souring of milk. The conversion is caused by enzymes of Streptococcus lactis and many other organisms. The first step is the splitting of lactose by a lactase in the bacterial cell wall. The glucose is converted to glucose-6-phosphate by the normal pathway. The galactose gets a phosphate added to form alpha-galactose-1-phosphate which is converted by another enzyme to glucose-6-phosphate. The two molecules of glucose-6-phosphate are then converted to lactic acid via several steps indentical those in the formation of lactic acid in muscle.

Milk Lipids (milk fats)
Fats in milk are called butterfat and occur as suspended globules which are easily seen via low power microscopes. The globules vary in size with the largest begin seven times the diameter of the smallest. These large globules tend to rise to the top and the yellowish color substance is called cream. When water is added the separation between milk and cream is sharper and faster. Therefore farmers used "water separators" and drained off the dilute milk from the bottom for pig feed and saved the cream to take to town.

Human and goat milk seldom gives rise to a layer of cream and farmers said goat milk had no cream. However, when water is added to human or goat milk a layer of cream becomes visible. The smaller globules of fat in human and goat milk do not rise as rapidly as he larger globules in cow mik. By forcing cow milk through small openings the large globules are broken up and the tiny globules do not rise noticeably and the product is called homogenized milk. Holstein milk has about 3% fat but Jersey milk has 5% fat. The fat content for individual cows ranges from 1% to 9%.

The fat content of cream is about 35% or less. The fat content of butter is 80% or more. Like other fats, milkfat is composed of several triglycerides. Oleic and Palmitic acids predominate. Human milk has more oleic acid and less of the short fatty acids (butyric, caproic, capryllic, and capric), but goat has more the fatty acids named for goat.